Recent advances in the fields of wireless communications, smart antennas, digital signal processing, and VLSI make it possible to provide a very high data rate channel at a physical layer of a wireless communications network. These technologies offer at least an-order-of-magnitude larger data rate than is currently available.
The open system interconnection (OSI) model defines the application, presentation, session, transport, network, data link, and physical layers. The data link layer includes a logical link control (LLC) layer and a media access control layer. The MAC layer controls how to gain access to the network, and the LLC layer controls frame synchronization, flow control and error checking. The physical layer transmits signals over the network. The invention is concerned with the data link and physical layers.
The “IEEE 802.11n PAR: Draft Amendment to STANDARD for Information Technology-Telecommunications and information exchange between systems-Local and Metropolitan networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Enhancements for Higher Throughput” specifies data rates up to 100 Mbps at the MAC layer. The “IEEE P802.15.SG3a PAR: amendment to Standard for Tele-communications and Information Exchange Between Systems—LAN/MAN Specific Requirements: Higher Speed Physical Layer Extension for the High Rate Wireless Personal Area Networks (WPAN)” specifies data rates of 110 Mbps or higher based on ultra-wideband (UWB) communications for personal area networks (PAN).
However, to deliver 100 Mbps throughput above the MAC service access point (SAP), a pure physical layer solution is insufficient, due to a substantial protocol overhead caused by the current protocol for the MAC layer. Therefore, the current MAC layer protocol must be improved to support a higher bandwidth.
Frame Formation
As shown in FIG. 1 for a transmitter 100 in a wireless local area networks (WLAN) designed according to the IEEE 802.11 standard, each MAC service data unit (MSDU) or frame 111, received from a logic link control layer (LLC) 110, is appended with a MAC header and a frame check sequence (FCS) trailer, at the MAC layer 120, to form a MAC layer protocol data unit (MPDU) or frame 121. At the physical layer, the MPDU is received as a physical layer service data unit (PSDU) or frame 122. At the physical layer 130, a physical layer convergence procedure (PLCP) header, a PLCP preamble, and tail and pad bits are attached to the PSDU frame 122 to form a physical layer protocol data unit (PPDU) or frame 131 for transmission on the channel.
FIG. 2 shows a format 200 for the MPDU frame 121 at the media access control (MAC) layer 120, and FIG. 3 shows a format 300 of the PPDU frame 131 at the physical (PHY) layer 130. The PPDU frame includes PLCP preamble 311, signal 312, and data fields 313. The details of the other fields in these formats are specified in the standard documents.
Frame Transmission
Networks designed according to the IEEE 802.11 standard utilize a distributed coordination function (DCF), and a point coordination function (PCF) to regulate channel access. The DCF applies in both infrastructure and ad-hoc modes and follows the well-known MAC paradigm of CSMA/CA. Before each packet transmission, a transmitting station senses the channel and waits until the channel becomes idle. Then, the station defers for a time interval of DCF inter-frame space (DIFS), enters a backoff stage, and determines a random time interval called backoff-time. The backoff-time is uniformly distributed between zero and contention window (CW) size. After the backoff timer expires, only one frame is transmitted over the channel, followed by an ACK message from the receiving station. Frames that are broadcast to all stations are not acknowledged. To reduce the probability of collisions, the size of the CW is increased after each perceived collision, until a maximum CW value is reached. The CW is reset to a fixed minimum value after a successful transmission of a frame.
Bandwidth is a scarce resource in a wireless network. For a high throughput WLAN according to the IEEE 802.11n standard requirement, the MAC protocol must achieve an efficiency of 70-80% to meet the design requirement of a bit rate of 100 Mbps at the MAC service access point (SAP). The overhead associated with frame transmission according to the current IEEE 802.11 standard wastes bandwidth. If each frame is acknowledged individually, then the following items represent significant overheads for a frame transmission: the MAC header, the physical layer header (PLCP header), the PLCP preamble, the backoff, the DIFS time, the SIFS time, and the ACK message.
It is desired to reduce this overhead so that the usable bandwidth on a wireless channel can be increased.